1. Technical Field
This disclosure generally relates to light emitting apparatuses, optical scanning apparatuses, and image forming apparatuses. In particular, the present invention relates to a light emitting apparatus, an optical scanning apparatus, and an image forming apparatus having improved heat-dissipating characteristics.
2. Description of the Related Art
A vertical-cavity surface-emitting laser (VCSEL; hereafter referred to as a surface emitting laser) emits laser light in a direction perpendicular to a semiconductor substrate. Compared with the edge-emitting semiconductor laser that emits laser light in a direction parallel to the semiconductor substrate, the surface emitting laser costs less, consumes less power, performs better, and is smaller in size. Thus, the surface emitting laser is gaining increasing attention in recent years as a light source for optical communications and or for the writing in printers, for example.
Particularly, the use of the surface emitting laser enables a 2-dimensional array structure of semiconductor laser elements, which has not been possible with the edge-emitting semiconductor laser. The 2-dimensional array structure of the surface emitting laser elements allows a high-density integration of light sources, thereby enabling an oscillation of high-output laser light.
When the surface emitting laser elements are arrayed 2-dimensionally, a large amount of heat is generated from a small area, thus requiring sufficient heat dissipation. If heat dissipation is insufficient in the surface emitting laser, the temperature of each surface emitting laser element increases. As a result, it becomes harder for induced emission of carriers to occur in the individual surface emitting laser elements, leading to a decrease in emission efficiency. Oscillation wavelengths may also fluctuate, or the lifetime of the element itself may become shorter.
Thus, in order to prevent the temperature increase in the surface emitting laser elements, not only are the characteristics of the surface emitting laser per se important, but also the package of the surface emitting laser should have high durability and superior heat-dissipating characteristics.
In this respect, the ceramic package offers superior durability, heat-dissipating characteristics, and heat and corrosion resistance over the organic package made of organic material such as resin. Thus, the ceramic package is suitable for mounting the surface emitting laser elements.
Japanese Laid-Open Patent Application No. 2004-228549 discloses a ceramic package for the mounting of the light-emitting elements, such as conventional surface emitting laser elements. The ceramic package includes a cavity (hereafter referred to as an “opening portion”) at a central portion on an upper surface of the package, in which the light-emitting elements is located. At a substantially central portion of a bottom surface in the opening portion, a conductor layer is formed by metallization to provide a mount portion on which the light-emitting elements are bonded. Areas around the conductor layer as the mount portion are metalized (in a metal wire forming step) to form a conductor layer for wire bonding.
Such a ceramic package may be manufactured by a ceramic green sheet laminating method which may involve the following steps. First, green sheets having punched-out portions are laminated so that the opening portion can be formed, thereby obtaining a laminated body. The bottom surface in the opening portion of the laminated body is coated with a conductive paste for forming a metalized layer (metal wiring layer) by the screen printing method, for example. The green sheet thus coated with the conductive paste is then calcined at high temperature, obtaining a ceramic sintered body having a metalized layer.
When a light emitting apparatus is constructed by mounting the surface emitting laser in the above ceramic package, there have been the following problems. Namely, when the surface emitting laser is mounted in the mount portion of the above-described ceramic package, heat dissipation is solely dependent on the ceramic material with the superior heat-dissipating characteristics compared with the organic package using a resin, and on a common electrode in the mount portion. No other measures are taken to actively improve heat-dissipating characteristics, resulting in insufficient heat dissipation.
The aforementioned common electrode is a single electrode combining the lower electrodes of the surface emitting laser in which plural surface emitting laser elements are arranged. When the surface emitting laser is mounted in the ceramic package, the common electrode is typically disposed on the bottom side and electrically and thermally connected to a ground terminal of the ceramic package by soldering or using an electrically conductive adhesive. Thus, in this case, the common electrode of the surface emitting laser is electrically connected to ground via the ground terminal of the ceramic package and ground wiring. However, the ground terminal and ground wiring have limited cross-sectional areas which limit the amount of heat generated by the surface emitting laser that can be dissipated via the ground terminal and ground wiring.
In addition, when the ceramic package with the surface emitting laser is mounted on a printed board, as the temperature rises and falls repeatedly corresponding to the turning on and off of light emission, a repetitive stress is produced by the difference in the coefficients of thermal expansion between the ceramic package and the printed board. The stress is concentrated at around the terminal electrodes between the printed board and the ceramic package that are soldered, resulting in a destruction of the soldered portion.
Furthermore, when the surface emitting laser is used as a light source for optical communications or for the writing in a printer, the light-emitted direction may vary, necessitating individual adjustments when combined with optical components, thus lowering productivity.
In the ceramic package formed by the aforementioned green sheet laminating method, the back surface, the mount surface, and the upper surface are formed by the green sheet surfaces, where parallelism can be ensured. The surface emitting laser has the feature that it emits light in a direction perpendicular to the semiconductor substrate surface. Thus, when the surface emitting laser is mounted on the mount portion provided on the bottom surface in the opening portion of the ceramic package, the emitted light is substantially perpendicular to the bottom surface or the upper surface of the ceramic package.
However, when a penetrating electrode is formed as a heat-dissipating electrode penetrating the ceramic package from the bottom surface in the opening portion to the back surface thereof, areas around the penetrating electrode are raised by the process of forming a via hole in the green sheet, thus producing surface irregularities. If the surface emitting laser is mounted on such a raised portion, the surface emitting laser is inclined, so that the direction of emission of light is also inclined with respect to the normal to the upper surface of the ceramic package.